Three-mating often occurs when three Elves have a very close relationship and share everything, including soul names. It's first proposed by Savah to Rayek to share Leetah with Cutter but he wouldn't think about it.
Known three-matings[]
-Goodtree, Lionleaper and Acorn
-When Rayek abducted Leetah and his family, Cutter joined with Nightfall and Redlance in the interim.